How to enable denoising for my final render?
Make sure the dropdown on the top right of the Image Viewer is set to the correct pass (DENOISED):



I'd suggest render to OpenEXR Multilayer to get both passes (beauty and denoised) in one file for comp:



Alternatively you can add a Render Layer node in the Compositor to get the DENOISED socket and connect both passes to a File Output node or even blend between both passes before saving:



Note: The Composite Node has only one input socket so this approach does not store any raw data.